The present invention relates to electrical terminals for use in photovoltaic solar panels and more particularly, to a terminal having improved mechanical strength.
The manufacture and use of photovoltaic solar cells are well known. It has become common practice to package large numbers of such solar cells in series or parallel strings to form panels or modules. Such panels have become standard commercial products. Methods for manufacturing such panels are disclosed in U.S. Pat. No. 4,067,764 issued to Walker et al. on Jan. 10, 1978 and U.S. Pat. No. 4,224,081 issued to Kawamura et al. on Sept. 23, 1980. As shown by these patents, it is standard practice to use one or more clear glass or structural plastic sheets as a superstrate to which the light receiving faces of solar cells are mounted. The cells are normally sandwiched between layers of relatively soft plastic materials such as polyvinylbutyral (PVB) or silicone resin. These soft laminating materials bond the cells to the superstrate and simultaneously act as a soft resilient protective layer which relieves stresses and protects the relatively brittle cells from mechanical forces. A protective layer is typically bonded over the back of the panel to provide protection from the environment including both weather and mechanical forces.
In manufacture of such solar panels, it is necessary that at least two electrical terminals be provided for making contact with the string of solar cells within the panel. With reference to FIG. 1, there is illustrated a prior art electrical terminal arrangement. FIG. 1 is an exploded cross-sectional view of the electrical terminal region of a solar panel with the light receiving face shown at the bottom. The lowermost layer 10 is a glass sheet which also forms the superstrate or primary mechanical structure for the panel. Immediately above glass plate 10 is a first layer 12 of PVB. A metal plate 14, typically made of tinned copper is positioned above layer 12. A second layer 16 of PVB is positioned above plate 14. An outer protective layer 18 is positioned on top of PVB layer 16. A cylindrical metal stud 20 is positioned within apertures 22 in layers 16 and 18. A lower end 24 of stud 20 is soldered to plate 14. Stud 20 is internally threaded and a bolt 26 is provided for attaching external leads to stud 20. A metal ribbon 28 is soldered to an edge of terminal plate 14 and extends between laminate layers 12 and 16 to make contact to solar cells within the module.
In the prior art structure, the stud 20 was soldered to terminal plate 14 by an induction heating technique performed after the lamination process was complete. This arrangement greatly facilitated the lamination process since the various layers 10, 12, 14, 16 and 18 could be assembled to an essentially flat structure before the application of heat and pressure to form the completed panel. However, it has been found in practice that various oxides and/or fluorides attack the solder connecting stud 20 to plate 14. Even though all terminals undergo mechanical strength tests, failures were encountered after aging both under field conditions and relatively protected storage conditions. When the junction between stud 20 and plate 14 fails, the entire panel becomes essentially useless and must be replaced.